This weekend, she had a thing for a friend's birthday at some bar. She posted a bunch of pictures of her and some other women doing the things women do when they go out to the bar. Then, her ex-boyfriend posted two pictures from the night on her wall. Selfies he took. One with his arm around her, and one of them kissing.
That's not what I'm angry about. She's an adult. If she wants to kiss her ex-boyfriend I couldn't give less of a shit. I'm angry because as far as I can tell, he crashed the party and made a lame-ass attempt to get back with her. She looked like she was tolerating it rather than enjoying it. Essentially, she put up with it because she was too nice to tell him to fuck off.
Like others said, it sounds like you are making a lot of assumptions about a situation you know next to nothing about. This is a major problem with young people who interact primarily online in that on social media sites you are only given a glimpse of what is going on in another person's life, so your imagination fills in the rest. If you are an especially negative person, you assume all sort of negative things which may or may not be true.
My advice is to stop fixating on this one girl. You are in college. There are plenty of women out there to chose from! Ask out other women. Go on a few dates. Work on improving your social skills. The best way to be less anxious about dating is to have plenty of options available.